1. Field of the Invention
The present invention relates to a focal-plane shutter used for a camera, which sequentially operates a leading blade group and a trailing blade group to be able to perform exposure by slit forming blades of the two blade groups.
2. Description of the Related Art
A focal-plane shutter which is configured to sequentially operate a leading blade group and a trailing blade group to perform exposure by slit forming blades of the two blade groups is widely adopted for a single-lens reflex image pickup system or the like (see Japanese Patent Laid-open No. 2006-221054). In such a focal-plane shutter, a leading blade driving member and a trailing blade driving member which are biased by a leading blade driving spring and a trailing blade driving spring respectively are configured to start rotating at a predetermined timing to perform exposure using the leading blade group and the trailing blade group (a first conventional configuration).
Here, a behavior when a change of the blade velocity occurs due to some kind of factors will be considered. Commonly, a leading blade velocity is not necessarily equal to a trailing blade velocity, but for the sake of shorthand, it is assumed that the leading blade velocity is equal to the trailing blade velocity in an initial state.
Even if the leading blade velocity and the trailing blade velocity change, an amount of exposure does not change if the relative velocity between the leading blade velocity and the trailing blade velocity is constant. However, the amount of exposure changes so as to be increased when the leading blade velocity is faster than the trailing blade velocity. On the other hand, the amount of exposure changes so as to be decreased when the leading blade velocity is slower than the trailing blade velocity.
When the leading blade velocity is slower than the trailing blade velocity, a slit that is to be formed by the leading blade group and the trailing blade group is not formed, which causes a phenomenon called a blade shut in which exposure for an imaging area is not performed. This phenomenon is fatal for a focal-plane shutter.
A focal-plane shutter disclosed in Japanese Patent Laid-open No. 2000-2907, in order to prevent the blade shut, a sliding contact area of the trailing blade driving member is configured to be larger than that of the leading blade driving member. In such a configuration, even if the blade velocity changes in a low-temperature environment, the generation of the blade shut is suppressed (a second conventional configuration).
However, in the first conventional configuration described above, the generation of the blade shut caused by a change of the blade velocity under a high-temperature or low-temperature environment or a change with the passage of time can not be suppressed. On the other hand, in the second conventional configuration, it is considered that the generation of the blade shut under a low-temperature environment is suppressed, but it is not considered that the generation of the blade shut in a case where the blade velocity changes in accordance with a change with the passage of time is suppressed.